GB1601817A - Plastics moulding process - Google Patents

Description

(54) PLASTICS MOULDING PROCESS (71) We, FORD MOTOR COMPANY LIMITED, of Eagle Way, Brentwood, Essex CMl3 3BW, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for the production of moulded plastics articles.

The present invention provides a process for the production of moulded thermosetting plastics articles, comprising forming a core by die-casting a metal alloy, the alloy having a melting point higher than the temperature required to cure a desired thermosetting plastics moulding compound but lower than the temperature at which said, compound when cured will be degraded, moulding said compound around the core to a desired shape, curing the compound, and heating the combination of core and moulded article to a temperature sufficient to melt the alloy, and in which the die casting is carried out such that the alloy solidifies adjacent the surface of the die but is not permitted to solidify in the centre of the casting space, whereby a hollow core is obtained.

The method of this invention may be practiced using any thermosetting plastics materials. The choice of material will depend upon the required characteristics of the moulded article. Suitable thermosetting plastics material include thermosetting polyester, epoxy, polyamide, phenolic, urea/formaldehyge, urea/butyraldehyde, resins and the like. Such materials are widely available from commercial sources and will be known to persons skilled in the art. The thermosetting plastics material will usually contain fillers, lubricants. mould release agents, cross limiting agents and other conventional additives in the usual amounts. Polyester thermosetting resins having a melting point of less than 130"C are preferred.

Any alloy having the required melting characteristics may be used in the method of the invention. Such alloys are readily commercially available and will be known to persons skilled in the art. For example a range of suitable alloys having melting points of from 20 to 292"C is manufactured by the Cerro Corporation and is available in Great Britain from Mining and Chemical Products Limited under the trade mark "Cerro". Such alloys include two or more of the elements Bismuth, tin, lead cadmium, indium, silver, copper, antimony, gallium and zinc in varying quantities.

Where polyester resins having a melting point of less than 130"C are used, alloys of tin and bismuth, which have melting points greater than the melting point of the resin, are preferred.

The core is formed by casting the alloy into a die, allowing the alloy to cool sufficiently to solidify the outer part of the alloy in the die, and removing the remainder of the alloy before it solidifies so that the resulting core is hollow. Preferably eutectic compositions are not used because eutectic cooling makes it difficult to achieve progressive solidification inwardly from the die walls.

Since the tin/bismuth alloy system has an eutectic point of 138"C, the preferred tin/bismuth alloys are those with melting points (liquids) in the range of 140"--170"C.

Preferably the molten alloy removed from the moulded article is used for forming further cores in a cyclic operation. In order to reduce energy expenditure during such cyclic operations the alloy is preferably kept within +25 of its melting point throughout the cycle.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of apparatus in accordance with the invention, with an indication of typical temperatures at various points in the process; Figure 2 is a more detailed view of the relationship between reservoir 10 of Figure 1 and die casting machine 13 of Figure 1.

Figure 3 is a plan view of an article moulded around a core as produced in the apparatus; and Figure 4 is a schematic view of part of the apparatus of Figure 1 showing a crosssection of the mould of Figure 2 taken along line 3-3.

Referring to Figures 1 and 2, a lagged and heated reservoir 10 contains a molten metal alloy, which will be discussed in greater detail hereinafter. Heating means, such as a burner 12 or an electrical heater, are provided to maintain the alloy at a temperature above its melting point. Molten alloy is transferred from the reservoir 10 to a low pressure die casting machine 13 by means of gas pressure, as described more fully with reference to Figure 2.

Referring to Figure 2, the die casting machine 13 comprises a lagged and heated feed manifold 30 which carries a removable two-piece mould 32. The feed manifold 30 can be heated by means of an electrical element 31 and is connected to the reservoir 10 by a feed line 33.

The reservoir 10 is hermetically sealed to the feed line 33 and to a gas line 35 which is connected to a source of compressed inert gas such as nitrogen.

The two-piece mould 32 also includes cooling means, e.g. a water or steam jacket 16 for controlling the temperature of the mould.

In use, the feed manifold 30 is initially heated to prevent the alloy from immediately solidifying on contract.

A pressurized inert gas, such as nitrogen is then fed into the reservoir 10, forcing molten alloy upwardly into the mould. Air displaced by the alloy leaves the mould 32 through a vent 38. When the mould is full of molten alloy the flow of gas into the reservoir is stopped and the nitrogen pressure is maintained constant for a period sufficient to allow the alloy immediately adjacent the walls of the mould to solidify. The gas pressure in the reservoir 10 is then reduced, so that the molten alloy in the centre of the casting flows back along the feed line 33 into the reservoir 10. The exact period of time for which the mould is kept full of alloy will vary with the temperature and cooling characteristics of the alloy and the mould 32.

In an alternative embodiment, the molten alloy is introduced into the mould 32 by means of an injection ram. At the end of the casting period, the ram is retracted and thus removing the molten alloy from the centre of the casting leaving a solidified hollow core.

Referring to Figure 2, the mould 32 is then opened and the core is removed therefrom by means of an overhead conveyor 18 which transfers the cores to a moulding area 20.

A number, in this embodiment three, of moulding presses 22 are provided, since the moulding process requires more than the die casting of the cores 14. Each moulding press 22 is of conventional construction and has a single ram under which a slidable table 26 may be moved between alternate positions.

A pair of mould members is provided, one for each position of the table. Thus, the pair which is free of the ram may be opened, a core from the conveyor 18 positioned therein and the mould pair closed ready for injection of moulding compound, while a similar assembly has the compound injected by the ram and held in position to allow curing to proceed.

The heat of the core may be sufficient to affect curing; if not heat may be applied in the usual way. Once the compound has cured to a sufficient degree to provide integrity, the core and the article moulded around it is removed from the mould and carried by the conveyor 18 into a melting tank 28. The combination of core 14 and article 24 therearound is illustrated in Figures 3 and 4, the article being, by way of example, part of an internal combustion engine inlet manifold.

The tank 28 contains oil maintained, as by an electric heater indicated at 29, at a temperature sufficient to melt the alloy of the core but not high enough to cause deterioration of the moulded article. The molten alloy collects at the foot of the tank 28, which is elevated, and from there is tapped from time to time via a conduit 27 back to the reservoir 10 through a heated feed tube by gravity or by means of a heated pump.

Using the apparatus illustrated in the drawings, inlet manifolds for internal combustion engines were manufactured from a bisphenol polyester thermosetting plastics material manufactured by Scott Bader Limited under the product reference number D600. The material has the following composition: Weight Component in Grams bisphenol polyester resin 1400 peroxide 65 hydrocarbon wax 1125 clay 1125 stearic acid 25 zinc stearate 72.5 glass fibers 850 styrene 350 The ingredients were mixed together at room temperature to form a dough and the dough is loaded into the moulds in the moulding presses 22. In an alternative process the thermosetting materials are formed into sheets and loaded into the moulds in sheet form.

The cores 14 were formed from a noneutectic alloy of tin and bismuth containing 47.5% tin and having a melting point of 145"C. In the casting process, the reservoir was maintained at 170"C.

Nitrogen at a pressure of 10 psi (gauge) was introduced into the reservoir 10 at a rate sufficient to fill the mould 32 within about seven seconds. The nitrogen pressure was maintained for a period of about 5 seconds, after which the pressure was released and the liquid from the centre of the mould was allowed to drain back into the reservoir 10.

The hollow core was removed from the die casting mould 32 at about 125"C and then transferred into the moulding presses 22 and the mould halves were closed around the cores at a temperature of 125 C (+4 C) and a closing pressure of 600 tons. The pressure and temperature within the mould initially causes the thermosetting material to flow into the spaces between the walls of the mould halves and the core, and then to undergo thermosetting into a final configuration.

After moulding, the core and moulded article are immersed in the oil in the tank 28, which is maintained at 170"C.

It will be appreciated that, throughout the moulding cycle, the alloy from which the cores are formed is not cooled more than 20"C below its melting point, nor is it heated to more than 25"C above its melting point, thus minimizing the required heat input of the system.

WHAT WE CLAIM IS: 1. A process for the production of moulded thermoset plastics articles, comprising forming a core by die-casting a metal alloy, the alloy having a melting point higher than the temperature required to cure a desired thermoset plastics moulding compound but lower than the temperature at which said compound when cured will be degraded, moulding said compound around the core to a desired shape, curing the compound, and heating the combination of core and moulded article to a temperature sufficient to melt the alloy, and in which the die-casting is carried out such that the alloy solidifies adjacent the surface of the die but is not permitted to solidify in the centre of the casting space, whereby a hollow core is obtained.

2. A process according to Claim 1, in which alloy at a predetermined temperature is injected upwardly into the die and held under pressure for a predetermined time sufficient to solidify the outer part of the alloy in the die, after which the central part of the alloy is allowed to drain from the die.

3. A process according to Claim 1 or 2, in which the moulding compound is polyester.

4. A process according to Claim 3, in which the moulding compound is a sheet moulding compound which is formed into strips or ropes prior to injection into the mould.

5. A process according to any one of claims 1 to 4, in which the alloy is a tinbismuth alloy.

6. A process according to Claim 5, in which the proportions of tin and bismuth are adjusted to give a melting point in the range 140--170"C.

7. A process according to any one of Claims I to 6, in which the molten alloy removed from the moulded article is recirculated for forming further cores, and in which the alloy temperature throughout the cycle is within melting point j25'C.

8. A process for the production of moulded thermoset plastics articles substantially as described, and as illustrated in the drawings.

9. A moulded article produced by a method according to any one of claims 1 to 8.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. The cores 14 were formed from a noneutectic alloy of tin and bismuth containing 47.5% tin and having a melting point of 145"C. In the casting process, the reservoir was maintained at 170"C. Nitrogen at a pressure of 10 psi (gauge) was introduced into the reservoir 10 at a rate sufficient to fill the mould 32 within about seven seconds. The nitrogen pressure was maintained for a period of about 5 seconds, after which the pressure was released and the liquid from the centre of the mould was allowed to drain back into the reservoir 10. The hollow core was removed from the die casting mould 32 at about 125"C and then transferred into the moulding presses 22 and the mould halves were closed around the cores at a temperature of 125 C (+4 C) and a closing pressure of 600 tons. The pressure and temperature within the mould initially causes the thermosetting material to flow into the spaces between the walls of the mould halves and the core, and then to undergo thermosetting into a final configuration. After moulding, the core and moulded article are immersed in the oil in the tank 28, which is maintained at 170"C. It will be appreciated that, throughout the moulding cycle, the alloy from which the cores are formed is not cooled more than 20"C below its melting point, nor is it heated to more than 25"C above its melting point, thus minimizing the required heat input of the system. WHAT WE CLAIM IS:

1. A process for the production of moulded thermoset plastics articles, comprising forming a core by die-casting a metal alloy, the alloy having a melting point higher than the temperature required to cure a desired thermoset plastics moulding compound but lower than the temperature at which said compound when cured will be degraded, moulding said compound around the core to a desired shape, curing the compound, and heating the combination of core and moulded article to a temperature sufficient to melt the alloy, and in which the die-casting is carried out such that the alloy solidifies adjacent the surface of the die but is not permitted to solidify in the centre of the casting space, whereby a hollow core is obtained.

2. A process according to Claim 1, in which alloy at a predetermined temperature is injected upwardly into the die and held under pressure for a predetermined time sufficient to solidify the outer part of the alloy in the die, after which the central part of the alloy is allowed to drain from the die.

3. A process according to Claim 1 or 2, in which the moulding compound is polyester.

4. A process according to Claim 3, in which the moulding compound is a sheet moulding compound which is formed into strips or ropes prior to injection into the mould.

5. A process according to any one of claims 1 to 4, in which the alloy is a tinbismuth alloy.

6. A process according to Claim 5, in which the proportions of tin and bismuth are adjusted to give a melting point in the range 140--170"C.

7. A process according to any one of Claims I to 6, in which the molten alloy removed from the moulded article is recirculated for forming further cores, and in which the alloy temperature throughout the cycle is within melting point j25'C.

8. A process for the production of moulded thermoset plastics articles substantially as described, and as illustrated in the drawings.

9. A moulded article produced by a method according to any one of claims 1 to 8.